The invention relates to a pyroelectric target; such a target is used, more especially, in infra-red radiation image pick up tubes.
It consists of a thin layer of an insulating material having the property of presenting a spontaneous electric polarization which depends on its temperature.
This property is turned to account for obtaining images. The heat radiation from the object falling on the target creates therein a relief of electric charges, which are ready by an electron beam in the manner of a vidicon. An electricity conducting coating is applied to one of the faces of the target for this purpose. It forms the signal plate of the target, from which the electric signals of the points are taken.
The invention also covers the tube in which this reading is carried out.
To increase the power of resolution of such a tube, it has been suggested, in the prior art, to give a reticular appearance to the target. Reference may be made, in this connection, more especially to French Pat. No. 71.17555 (U.S. Pat. No. 2,137,163).
The target consists of a mosaic of small islands made from a pyroelectric material, separated from each other, each of which forms a spot of the target, bonded to a support which is made conducting as has been mentioned. Such a structure improves the resolution of the target because of the obstacle which it puts in the way of lateral heat conductibility between spots, which obstacle means that each one of them is withdrawn from the thermal point of view from the influence of the others.
Such a structure presents a disadvantage.
The small islands, of small size, separated from each other present low adherence to the support. It will be noted in this respect that, once the target is finished, these small islands are, for example, cubes having sides of 30 to 40 micrometers, separated by gaps of about 10 micrometers. According to a currently used method of preparation, they are obtained by etching a wafer of the pyroelectric material, triglycine sulfate TGS for example, of a thickness of about 20 micrometers, after this latter has been bonded to the support, consisting of a sheet of plastic material transparent to the incident radiation and with low heat conductibility, stretched on a metal frame covered with the conducting coating, or signal plate, already mentioned. The thicknesses of the support and of the conducting coating are of a few micrometers and a few hundred angstroms respectively. The metal used for the coating being for example gold or chromium. The metal frame is a ring of a diameter of about 20 millimeters.
On the other hand, in order to improve the caracteristics of the target, there is sometimes provided in the prior art another electricity conducting layer, disposed on the bonding layer just below the pyroelectric material; this layer, electrically connected to the preceding coating, avoids the capacity effect of the bonding agent and improves the sensitivity of the target. This arrangement presents another disadvantage.
This second conducting layer, which in practice does not improve the mechanical strength of the small islands, requires on the other hand very high accuracy in the etching of the pyroelectric body if destruction of said layer is to be avoided during such etching and its effect reduced to nothing; this etching must however go as far as this layer if the loss of resolution is to be avoided which the presence of a remnant of pyroelectric material over the whole surface thereof would cause.
This is a second disadvantage.